Penetrating coatings or stains have traditionally been formed as oil based systems having inherently good spreading characteristics for facilitating their application to various wood surfaces.
Application characteristics for these coatings are well understood in the art and are important to assure uniform, protective and esthetic characteristics in the coated wood. In particular, it is desirable that penetrating coatings or stains be provided with suitable rheological characteristics such as "brush drag" for facilitating uniform spreading of the coating on the wood surface by means of a brush, roller, etc. Other characteristics such as drying rate are also important in achieving coating uniformity on the wood surface while also maintaining necessary protective characteristics of the coating after it dries.
As noted above, these various characteristics can readily be assured in oil based systems. However, more recently, water based or waterborne systems have been given greater consideration for a variety of reasons. As a primary consideration, under the Federal Clean Air Act, Air Pollution Control Districts have established local limits for volatile organic compounds. As an example, a maximum limit of 350 grams per liter of volatile organic compounds (VOC) has been established as a maximum limit at least in portions of the state of California, for penetrating coatings of the tYpe toward which the present invention is directed. In efiect, these statutory regulations establish an upper limit for the amount of organic solvents which can be present in such penetrating coatings.
Waterborne coatings or stains have thus become of greater interest as a means for meeting the statutory guidelines. At the same time, waterborne stains provide other desirable advantages which are also common to water based paints and are generally well known, such as ease of clean-up with water and low flammability.
A variety of water based or waterborne coatings are available in the prior art. They commonly employ glycol ethers of a wide variety as a coupling solvent or cosolvent which, together with an alkaline solubilizer or neutralizer, assures solubility of the resin component in the water.
Prior art water based penetrating coatings or stains have also been developed with a variety of resins which are usually of lower molecular weight in order to assure their penetration of wood substrate. Water based penetrating coatings or stains in the prior art commonly include lower molecular weight resins including maleinized linseed oils, alkyd resins, modified alkyds and the like which can be solubilized in water with conventional cosolvents such as the glycol ethers (and alkaline solubilizer) referred to above. For example, Gunter et al., U.S. Pat. No. 4,269,866 is representative of prior art for coating compositions containing a water-dilutable maleinized oil. Vasishth et al., U.S. Pat. No. 4,276,329 disclosed a coating composition for treating and protecting wood surfaces, the composition comprising a low molecular weight alkyd resin in a cosolvent of water and a small amount of glycol ether solvent.
Numerous additional prior art references also have disclosed water based systems. However, such prior art water based systems have generally been incapable of providing equally good application characteristics as the oil based systems referred to above.
Accordingly, there has been found to remain a need for improved penetrating coatings or stain systems. As was initially noted above, penetrating coatings generally include but are not limited to clear sealants, semi-transparent stains, solid color stains and deck stains. Each of these products exhibits certain unique characteristics. For example, clear sealants are specifically characterized by an essential absence of pigment. A semi-transparent stain is contemplated for covering and providing some coloration for the wood surface while allowing the texture and grain of the wood to clearly show through the stain or coating composition. A solid color stain is, in effect, a low solids penetrating paint which allows the wood texture to show through the coating; however, there is usually sufficient pigment to cover or hide the grain of the wood. Finally, a deck stain can be similar to a semi-transparent or solid color stain in terms of pigment quantity and which is relatively fast drying, capable of developing early water resistance and resulting in a durable coating for use on horizontal surfaces normally subject to abrasion from foot traffic and the like.
Additional characteristics of these various specific products are discussed in greater detail below. ln any event, a need has been found for improved waterborne penetrating coatings or stains which offer application characteristics more similar to application characteristics for oil based products.